the summer of 480 B.C., Xerxes massive Persian army had already
caused the Greeks to abandon plans to make a stand in Thessaly and
assorted Greek forces took up a position at Themopylae ("The
Warm Gates"), near Artimesium. The battle is mostly remembered,
however, as Sparta's moment of glory when the 300 Spartans under
Leonidas fought what was a suicidal (in no small part due to the
traitorous Ephialtes) delaying action which did buy the other Greek
cities some time.
actual date of the battle is uncertain, but is obviously tied to the
Spartan celebration of Karnea (held sometime in the Spartan month of
Karnea -- roughly August/September, which delayed the arrival of
reinforcements for Leonidas.